Field of the Invention
The present invention relates to a manufacturing method of a membrane electrode assembly (MEA). More specifically, the present invention relates to a method for manufacturing an MEA which is used in a polymer electrolyte fuel cell (PEFC), and a method for joining catalyst electrodes to an electrolyte membrane in the PEFC.
Description of the Related Art
A fuel cell which utilizes hydrogen and oxygen produces only water as a by-product in principle and is attracting attention as a power generation system which (little harms the environment) is environmentally friendly. In recent years, polymer electrolyte fuel cells (PEFCs) are especially considered as a promising fuel cell for a vehicle power source, and a stationary and household power supply etc. since the PEFCs, which use a proton conductive polymer electrolyte as an electrolyte, can operate at a low temperature, and have a high density output as well as being capable of downsizing.
PEFCs generally have a structure in which many single cells are stacked. A single cell has a separator on the anode, an electrode on the anode, a proton conductive polymer electrolyte membrane, an electrode on the cathode and a separator on the cathode in this order. The electrodes on the anode and cathode include an electrode substrate and a catalyst electrode, respectively.
In general, a product obtained by joining the catalyst electrodes on the anode and cathode to the polymer electrolyte membrane 3 is referred to as a membrane electrode assembly (MEA).
In manufacturing the MEA, a hot press is generally applied as a means for joining the catalyst electrodes and the polymer electrolyte membrane. The hot press has an advantage that the joining process is performed by relatively simple equipment. Whereas the hot press may cause a decrease in production efficiency because it is necessary to separate the hot press process from the catalyst electrodes forming process since the catalyst electrodes forming process is generally a continuous process while the hot press requires a step-by-step treatment (pressing only a single plate at a time) in principle. In addition, the hot press also has a problem that it is hard to form an even catalyst electrode on the polymer electrolyte membrane since an in-plane unevenness in pressing pressure is easily generated. In an extreme case, the catalyst electrodes and the polymer electrolyte membrane may be destroyed if an extremely high pressure is applied on a certain point.
In order to solve these problems, roll press methods and lamination methods are often proposed as a method for joining the catalyst electrodes and the polymer electrolyte membrane.
For example, a method for transferring portions of a catalyst layer formed on a long substrate film of a transfer film onto both surfaces of a long polymer electrolyte membrane intermittently with a predetermined interval is described in Patent Document 1 below. In addition, a method for weakening an adhesion force between a support medium and an object formed on the support medium by heating, irradiating activation light, elongating the support medium and/or irradiating ultrasound so that the object can be smoothly transferred in the case where an MEA is manufactured by transferring and stacking gas catalyst layers, diffusion layers and gaskets formed on a support medium is described in Patent Document 2 below.
<Patent document 1> JP-A-2006-185762
<Patent document 2> JP-A-2003-303599
The performance of the catalyst electrode depends on factors such as the type of material used, the amount of the catalyst, the structure and volume of micro pores, the water affinity and repellency, and the thickness etc. These factors can be controlled by changing the type of material, composition ratio of the materials, and production conditions. A change in the material, however, involves a number of influences on various manufacturing process steps of the fuel cell and a tremendous time and effort would have to be spent to grasp the all influences.
Furthermore, it is necessary to form two catalyst electrodes having different structures on both sides of the polymer electrolyte membrane since the required characteristics for catalyst electrodes of an MEA on the anode differ from those on the cathode. Thus if the required characteristics for the catalyst electrodes on the anode and the cathode are obtained, for example, by selecting each material (or material composition), it is necessary either to arrange two types of equipment or to use the same equipment twice because ultimately two types of catalyst electrodes have to be made. In which ever case, a heavy burden relating to the production expense or the production takt time must be bore.